<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title></title>
    <script>
    document.onclick=function (){
      let arr=[1,2,3];

      arr.a=()=>{
        alert(this);
      };

      arr.a();
    };
    </script>
  </head>
  <body>

  </body>
</html>
